Ferrous metal articles are subjected to corrosion when exposed to moisture, salt, and oxygen. Various types of protective coatings have been applied to ferrous articles in the past to resist corrosion. Initially, the protective coatings consisted of grease or oil but coatings of this type are of limited effectiveness. Subsequently, solvent-type coatings were employed in which a coating material was dissolved in an organic solvent. After application of the coating solution to the article, the solvent or carrier is evaporated to provide a firm and relatively non-greasy coating. However, solvent-type coatings have become less popular due to the environmental concerns in handling the solvent. Not only do solvent-type coatings require the use of expensive pollution control equipment, but the solvents provide a hazard to personnel.
Water-base coatings have also been employed which overcome the pollution problems associated with solvent-type coatings. However, water-base coatings have a serious disadvantage in requiring substantially longer drying times than the solvent-base counterparts.
Thermosetting resin coatings have also been utilized as protective coatings. However, these systems require the mixing of two reactive components and pose hazards in various degrees to personnel. In addition, the thermosetting resin coatings are relatively expensive and require the use of heat to accelerate the reaction between the components.
Hot melt dips have also been employed to provide protective coatings on ferrous articles. The use of a hot melt dip requires that large vats of molten material be maintained at high temperatures. Thus, the hot melt system requires substantial capital expenditures as well as emission control equipment. In addition, a hot melt dip provides difficulty in controlling the film thickness and there are also product stability concerns due to high temperature aging.
A further type of coating as used in the past is a sprayable hot melt coating. However, sprayable hot melt coatings require expensive equipment such as drum unloaders, heated fluid lines, and special spray guns to prevent solidification of the molten coating material.